BOAC, I have not seen anything but IRS on any aircraft I fly.
The original explanation was correct according to my books. IRS does the same thing as an INS, however the IRS uses laser gyroscopes in stead of mechanical ones. That is also the answer that the JAA-ATPL guys prefer
Or maybe we are both right and it comes down to semantics. But there certainly is no INS on the 320.
Further more, the IRS's deliver all the attitude data to the glass cockpit of the 320 and 737.